It is almost impossible for a nonlawyer to present a case successfully in the U.S.
legal system. Lack of representation so compromises a criminal defendant’s
rights that the U.S Supreme Court requires all criminal defendants to have
appointed counsel at government expense, if the defendant is unable to pay a
lawyer. [Gideon v. Wainwright, 372 U.S. 335 (1963)
] This means that the
defendant has to be informed of the right to have counsel, that if he says
anything before counsel arrives it can be used against him at trial, and that if
he cannot afford a lawyer, one will be appointed for him—his
Miranda rights. [Miranda v. Arizona, 384 U.S. 436, 16 L. Ed. 2d 694, 86 S. Ct. 1602 (1966)
.]
This does not mean that unless the defendant can afford the O.J. defense
team that the state will pay for his defense. The state is required only to pay
the defense lawyer for the minimum defense necessary to preserve the client’s
constitutional rights, not to win the client’s case. Other than cases involving
the death penalty, the state is usually unwilling to pay for expert testimony,
the evaluation of forensic evidence, investigators, or the other basic
requirements for properly defending complex criminal cases.

The police have to inform a potential defendant of his rights when it is probable
that he will be charged. They do not have to inform witnesses of their Miranda
rights during the routine investigation of case. If an investigator starts reading
the Miranda rights to you, you are in trouble and need to get an attorney
before you say anything else.